A thrust roller bearing supporting a thrust load is used in a part to which the thrust load is applied in an automatic transmission of a car. Recently, the thrust roller bearing is required to lower torque in view of lower fuel consumption as well as to be prevented from being damaged.
The thrust roller bearing has track rings such as an outer ring and an inner ring, a plurality of rollers, and a retainer retaining the rollers in general. FIG. 11 is a sectional view showing a part of a conventional thrust roller bearing. Referring to FIG. 11, a thrust roller bearing 101 comprises a plurality of needle rollers 103 arranged between a pair of track rings 102a and 102b and rolling on track surfaces of the track rings 102a and 102b, and a retainer 104 retaining the plurality of needle rollers 103. The retainer 104 is bent several times in a rotation axis direction of the retainer 104 such that its cross section has a roughly W shape. A flange part 105a extending in an axial direction is provided at an outer diameter side end of the track ring 102a. In addition, a flange part 105b extending in the axial direction is provided at an inner diameter side end of the track ring 102b. 
A technique regarding a thrust roller bearing having the same constitution as the above is disclosed in Japanese Unexamined Patent Publication No. 2006-170371. According to the Japanese Unexamined Patent Publication No. 2006-170371, a thrust roller bearing has a pair of track rings, needle rollers arranged between the track rings, and a retainer retaining the needle rollers. The retainer is bent in an axial direction so as to have a W-shaped cross section. Thus, a height in the axial direction of an outer diameter part and an intermediate part is smaller than a height in the axial direction of an inner diameter to lower torque.
In addition, according to Japanese Unexamined Patent Publication No. 2006-118562, a retainer provided in a thrust roller bearing is constituted such that one wholly annular element having a U-shaped cross section and the other wholly annular element having an inverted U-shaped cross section are combined. According to the Japanese Unexamined Patent Publication No. 2006-118562, a contact area between a track ring and the retainer in a radial direction is reduced to lower friction and to lower torque.
Referring to FIG. 11 again, during the operation of the thrust roller bearing 101, while the track rings 102a and 102b rotate together with the retainer 104, a rotation axis of the track rings 102a and 102b does not coincide with a rotation axis of the retainer 104 due to dimensional deviation and the like in some cases. In this case, the retainer 104 is sandwiched between the track rings 102a and 102b in the radial direction, more specifically, between the flange parts 105a and 105b of the track rings 102a and 102b, so that a load is applied in the radial direction from the track rings 102a and 102b to the retainer 104. Therefore, strength against the radial load is required to be high in the retainer 104.
Here, according to the Japanese Unexamined Patent Publication No. 2006-170371, the retainer is provided by bending a thin plate member several times so as to have the W-shaped cross section. Since the radial strength is low in such retainer, the retainer could be damaged and deformed in the above case.
In addition, according to the Japanese Unexamined Patent Publication No. 2006-118562, since the retainer is composed of the plurality of members, they could be separated during the operation of the thrust roller bearing.
Furthermore, according to the Japanese Unexamined Patent Publication No. 2006-170371 and the Japanese Unexamined Patent Publication No. 2006-118562, the retainer provided in the thrust roller bearing comes in contact with the track ring in a thickness direction in the operation state of the thrust roller bearing. In this case, torque could be increased due to the contact between the retainer and the track ring in the operation state of the thrust roller bearing.